I can reproduce only in non-e10s windows.
Created attachment 8733426 [details] [diff] [review] Fix
https://hg.mozilla.org/integration/fx-team/rev/3466c5a966966230a9347bb31ced93b1985f529c Bug 1258019 - avoid JS errors on closed windows in ContentWebRTC.jsm, r=felipe.
I have reproduced this bug with Aurora 48.0a1 (2016/03/18) on Windows 10, 64 bit! The Bug's fix is now verified Beta 48.0b9. Build ID 20160718142219 User Agent Mozilla/5.0 (Windows NT 10.0; WOW64; rv:48.0) Gecko/20100101 Firefox/48.0 [bugday-20160727]